A common process for producing polyolefin polymers is a gas phase polymerization process. For any polymerization process, the catalyst system employed is typically of great importance. A catalyst system generally includes at least one catalyst and at least one co-catalyst. Organometallic compounds such as metal alkyls are well known for use in this area. They are commonly used as “cocatalysts” (or catalyst “activators”) with Ziegler-Natta catalysts. Examples of cocatalysts include the use of triethylaluminum (TEAl) and trimethylaluminum (TMA).
Metal alkyls have also been used with advanced catalysts such as metallocene catalysts. With metallocene catalysis, the metal alkyl has at least two roles: (1) activating the catalyst; and (2) eliminating impurities from the reaction medium. With respect to activation of the metallocene catalyst, unlike Ziegler-Natta catalysts as referenced above, the common, low molecular weight metal alkyls, such as TEAl, TMA, and DEZ, are not effective in activating metallocene catalysts. Rather, high molecular weight metal alkyls, such as methylaluminoxane (MAO), are often used.
For example, U.S. Pat. No. 5,324,800 discloses the use of MAO with metallocene catalysts. Low molecular weight metal alkyls may be used to scavenge impurities such as moisture and oxygen from the reaction medium. This has the effect of eliminating the catalyst poisons from the system and thereby maximizing the catalyst productivity.
WO 1996/008520 discloses metallocene catalysis using less than 300 ppm of an organometallic scavenger for reactor start-up, and then discontinuing the introduction of the scavenger (or reducing the rate of introduction) such that the concentration of oligomers in the product is maintained at less than 50 ppm by weight.
EP 7 81 300 discloses a continuous polymerization process with metallocene catalysts using less than 50 ppm of an organometallic scavenger based on bed weight.
U.S. Pat. No. 5,712,352 discloses a metallocene polymerization process using less than 30 ppm of an organometallic scavenger. The patent also describes the introduction of scavenger during the start-up process with the subsequent removal at least 95% of the scavenger prior to the introduction of catalyst. Additionally, the patent describes the problems that may occur when too much scavenger is used such as, for example, generation of fines in the fluid bed and the production of high levels of C14-C18 oligomers in the resin product.
U.S. Pat. No. 5,763,543 discloses a metallocene polymerization process using less than 300 ppm of an organometallic scavenger for reactor start-up, and then discontinuing the introduction of scavenger once the catalysts productivity reaches 2500 or higher.
In addition to choosing desirable components for a catalyst system, reactor start-up is an important aspect for reactor continuity and operability. For example, during a gas phase polymerization process, a fluidized bed reactor may contain a fluidized dense-phase bed including a mixture of reaction gas, polymer (resin) particles, a catalyst system, and optionally, catalyst modifiers or other additives. Before such a polymerization reaction begins, a “seed bed” is typically loaded into the reactor, or is present in the reactor from a previous polymerization. The seed bed typically consists of granular material that is or includes polymer particles. The polymer particles need not be identical to the desired end product of the reaction.
For example, U.S. Patent Application Publication No. 2007/0073012 discloses a method for preparing a reactor for performance of a polymerization reaction in the reactor, said method including the steps of: (a) loading a seed bed into the reactor; and (b) loading at least one continuity additive into the reactor. Examples of the at least one continuity additive are aluminum stearate, other metal stearates, and ethoxylated amines. Such methods have improved the efficiency and operability of the polymerization reaction especially during the critical initial stage(s) of a polymerization reaction (before the reaction has stabilized).
However, further improvements in efficiency and operability of the polymerization reaction are needed. Particularly, there is a continued need to address the vulnerability of the reactor to sheeting and/or fouling during the critical initial stage(s) of the polymerization reaction.
Sheeting is a phenomenon during which catalyst and resin particles adhere to the reactor walls or a site proximate the reactor wall possibly due to electrostatic forces. If the catalyst and resin particles remain stationary long enough under a reactive environment, excess temperatures can result in particle fusion which in turn can lead to the formation of undesirable thin fused agglomerates (sheets) that appear in the granular products. The sheets of fused resin vary widely in size, but are similar in most respects. They are usually about ¼ to ½ inch thick and are about 1 to 5 feet long, with some sheets being even longer. Sheets may have a width of about 3 to 18 inches or more. The sheets are often composed of a core of fused polymer that may be oriented in the length dimension of the sheets and their surfaces are covered with granular resin fused to the core. The edges of the sheets can have a hairy appearance from strands of fused polymer.
In gas phase reactors, sheeting is generally characterized by the formation of solid masses of polymer on the walls of the reactor. These solid masses of polymer (e.g., the sheets) eventually become dislodged from the walls and fall into the reaction section, where they interfere with fluidization, block the product discharge port, plug the distributor plate, and usually force a reactor shut-down for cleaning, any one of which can be termed a “discontinuity event”, which in general is a disruption in the continuous operation of a polymerization reactor. The terms “sheeting, chunking and/or fouling” while used synonymously herein, may describe different manifestations of similar problems, in each case they can lead to a reactor discontinuity event.
There are at least two distinct forms of sheeting that occur in gas phase reactors. The two forms (or types) of sheeting are described as wall sheets or dome sheets, depending on where they are formed in the reactor. Wall sheets are formed on the walls (generally vertical sections) of the reaction section. Dome sheets are formed much higher in the reactor, on the conical section of the dome, or on the hemi-spherical head on the top of the reactor.